CORAL 605 



red and of black coral are quite unessential, the chief being that black coral contains a 

 lai'ger proportion of organic material, which also probably differs in character from that 

 present in red coral. 



The colour of black coral is no doubt due to the presence of this organic material ; it 

 has, indeed, been referred to the presence of manganese dioxide, but this substance was not 

 detected in the above analysis. It has also been suggested that the action of hydrogen sulphide 

 upon red coral might result in a change of colour from red to black ; but this requires 

 proof. Attempts to refer the red colour of fresh coral to the presence of inorganic con- 

 stituents have been equally unsuccessful. It has been said to be due to the presence of iron 

 oxide, of which Tischer found r720 and other chemists up to 4!"75 per cent. This, however, 

 is not very probable, seeing that the red colour is destroyed on ignition, just as is the black. 

 The red colouring matter is thus presumably organic in nature, as is the pigment of variously 

 coloured univalve and bivalve shells, and the change of red coral first to black and then to 

 dirty yellow mark different stages in an oxidation pi'ocess. 



The Living Coral. — As already stated, the branching axis of a living coral-stock is 

 invested by the coenosarc, a layer of soft, red, living material, with a surface like velvet, 

 and a thickness of less than a line. In the substance of the coenosarc are embedded 

 calcareous spicules, which can be felt by pressing a portion of it between the thumb and 

 finger. They are so small, however, that their form can only be made out by examining 

 thin sections under the microscope. The coenosarc extends beyond the ends of the branches 

 of the skeleton, and the blunt prolongations of the branches so formed are soft and flexible, 

 and can be cut through with a sharp knife, while the portion containing the calcareous 

 skeletal rod cannot be so treated. The foot of young colonies is invested with coenosarc, 

 just as are the trunk and the branches, but in old stocks, not only the foot, but the lower 

 part of the stem and the lower branches, are bare and often much corroded and eaten 

 away. When a branch of living coral is allowed to dry in the air, the coenosarc assumes 

 the appearance of dry, rough skin of a brick-red or red-lead colour, raised here and there 

 into wart-like protuberances. From the central orifice of each protuberance radiate eight 

 short grooves, which divide it into eight wedge-shaped portions, and give the surface of the 

 wart the appearance of an eight-rayed star. 



These little protuberances mark the spots occupied by the individual polyps of which 

 the colony is built up. Nothing more than what has been described can be seen of the 

 polyps when a branch of living coral is examined in water which has been disturbed or is 

 in motion, and the branch appears of a uniform red colour. When the water has come to 

 rest, however, one may watch the wart-like prominences gradually expand and open, and the 

 white cylindrical bodies of the polyps, each with eight white, pinnate tentacles come into 

 view. The position of each polyp is marked in the skeleton by a shallow depression, to 

 which attention has been directed ah-eady. The polyps respond readily to external irrita- 

 tion ; no matter how slightly the water be disturbed, or how softly one of their number be 

 touched, each one is instantly retracted into its pocket-like depression in the coenosarc, and 

 then reassumes its original aspect of a wart-like protuberance. It may be some hours 

 before the polyps may again be seen in a fully expanded condition, and even then they 

 expand, only to retract again on the slightest disturbance of the surrounding medium. 

 When fully expanded, the coral-polyps look like white starry blossoms on a coral-red back- 

 ground, and, indeed, formerly the coral-stock was regarded as a plant of which the polyps 

 were the flowers. The true nature of the organism was first recognised, in the year 1723, 

 by the French physician and naturalist, Peyssonel, whose perspicacity was not, however^ 

 recognised by his brother zoologists. 



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